1. Field of the Invention
This invention relates to a temperature control system for controlling the temperature of a sensor element of a humidity sensor that is arranged in an exhaust pipe of an internal combustion engine for detecting humidity within the exhaust pipe.
2. Description of the Prior Art
There has been proposed an internal combustion engine having an adsorbent arranged in an exhaust system thereof, for adsorbing hydrocarbons in exhaust gases. The adsorbent carries zeolite on its surface. When exhaust gases are passing through the adsorbent, the hydrocarbons contained therein enter small holes of the zeolite, thereby being adsorbed by the adsorbent. The adsorbent of this kind desorbs i.e. releases hydrocarbons once adsorbed thereby when it is heated by exhaust gases to a temperature equal to or higher than a predetermined temperature (e.g. 100 to 250xc2x0 C.). The desorbed hydrocarbons are recirculated to the engine e.g. via an EGR pipe. The adsorbent repeatedly carries out the adsorption and desorption of hydrocarbons as described above. However, the amount of hydrocarbons which could not be desorbed and permanently remain in the adsorbent may progressively increase or the small holes of the adsorbent may be destroyed by a long-term use thereof. This results in the degradation of the adsorbent to lower an adsorbing capacity of the adsorbent for adsorbing hydrocarbons. This makes it necessary to detect the degree of degradation of the adsorbent.
The present assignee has already proposed a degradation-detecting device for detecting the above degradation of an adsorbent e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 2001-323811. In this degradation-detecting device, attention is paid to proportionality between the adsorbent""s capabilities of adsorbing hydrocarbons and of adsorbing moisture, and the humidity of exhaust gases having passed through the adsorbent is detected by the humidity sensor while the adsorbent is adsorbing hydrocarbons, to determine lowering of the adsorbent""s capabilities of adsorbing hydrocarbons and moisture, that is, the degradation of the adsorbent, based on the detected humidity. The humidity sensor has a sensor element formed by a porous body having a large number of small holes. When exhaust gases is passing through the sensor element, moisture in the exhaust gases enters the small holes of the porous body, and is adsorbed by the same, based on which the humidity of exhaust gases is detected. As described above, the sensor element is exposed to exhaust gases in detecting humidity of the exhaust gases, and hence water droplets produced by condensation, and impurities, such as unburned components of fuel, contained in the exhaust gases, are attached to the sensor element. In this case, the humidity of the exhaust gases cannot be detected properly, and the degree of degradation of the adsorbent cannot be detected properly, either.
To overcome this problem, in the proposed degradation-detecting device, the temperature of the sensor element is controlled by heating the same by using a heater to eliminate water droplets attached thereto and suppress attachment of water droplets thereto, whereby an excellent detection accuracy of the humidity sensor is maintained. More specifically, in the temperature control of the humidity sensor, the heater is operated for a predetermined time period, judging that condensation has been formed on the sensor element, either when the intake air temperature is lower than a predetermined temperature before the start of the engine, or when idling of the engine has continued for a predetermined time period or longer after the start of the engine.
In the temperature control of the above humidity sensor, the heater is operated for the predetermined time period only under circumstances in which it is presumed that condensation has occurred. However, the above determination of degradation of the adsorbent is carried out while hydrocarbons are being adsorbed by the adsorbent, and hence impurities in exhaust gases other than water droplets may be attached to the humidity sensor. Even if such impurities have been attached, so long as it is not presumed that condensation has been formed, the above temperature control is not carried out, resulting in the degraded detection accuracy of the humidity sensor. Further, since the heater is simply operated for the predetermined time period, there is a fear that the impurities cannot be sufficiently eliminated, e.g. depending on a temperature condition of the exhaust system. Moreover, since the sensor element is suddenly heated by the heater from a state having condensation formed thereon, the temperature of the sensor element sharply rises from a low temperature, which can cause cracking of the sensor element.
It is an object of the invention to provide a temperature control system for a humidity sensor, which is capable of eliminating impurities attached to a sensor element of the sensor efficiently and sufficiently while preventing the sensor element from being cracked due to heat generated by a heater in a state of condensation formed thereon, and suppressing attachment of impurities to the sensor element, thereby making it possible to preserve an excellent detection accuracy of the humidity sensor.
To attain the above object, the present invention provides a temperature control system for controlling a temperature of a sensor element of a humidity sensor arranged in an exhaust pipe of an internal combustion engine for detecting humidity within the exhaust pipe, the temperature control system comprising:
a heater for heating the sensor element;
temperature-detecting means for detecting the temperature of the sensor element; and
heater control means for causing the heater to operate when the temperature of the sensor element detected by the temperature-detecting means is higher than a first predetermined temperature.
According to this temperature control system for a humidity sensor, the heater control means causes the heater to operate when the temperature of the sensor element detected by the temperature-detecting means is higher than a first predetermined temperature, whereby the sensor element is heated. Thus, the heater is operated when the sensor element is in a high temperature condition, so that it is possible to efficiently burn and eliminate impurities attached to the sensor element. This makes it possible to properly restore detection accuracy of the humidity sensor.
Preferably, the first predetermined temperature is a temperature at which condensation cannot be formed on the sensor element.
According to this preferred embodiment, the heater is operated when the temperature of the sensor element is higher than a temperature at which condensation cannot be formed on the sensor element, and hence the sensor element can be heated in a state where no condensation has been formed on the sensor element. Therefore, it is possible to prevent the sensor element from being cracked by heat generated by the heater when it has condensation formed thereon.
Preferably, the heater is configured to generate a variable amount of heat, and the heater control means causes the heater to operate to generate a smaller amount of heat when the temperature of the sensor element is equal to or lower than the first predetermined temperature, than when the temperature of the sensor element is higher than the first predetermined temperature.
According to this preferred embodiment, the heater configured to generate a variable amount of heat is operated to generate a smaller amount of heat when the temperature of the sensor element is equal to or lower than the first predetermined temperature, than when the temperature of the sensor element is higher than the first predetermined temperature. This causes the temperature of the sensor element to slowly rise from a low temperature condition in which condensation has been formed on the sensor element, thereby making it possible to eliminate condensation with minimum power consumption while positively preventing the sensor element from being cracked by a sudden change in temperature.
Preferably, the temperature control system further comprises atmosphere-determining means for determining whether or not an atmosphere in which the humidity sensor is operating is in an oxidizing condition, and the heater control means causes the heater to stop operating when time over which the heater operates with the temperature of the sensor element being higher than a second predetermined temperature high enough to eliminate impurities attached to the sensor element and the atmosphere in which the humidity sensor is operating being in the oxidizing condition has reached a predetermined time period.
According to this preferred embodiment, the heater is stopped from operating when a cumulative time period over which the heater operates with the temperature of the sensor element being high enough to eliminate impurities attached to the sensor element and the atmosphere in which the humidity sensor is operating being in the oxidizing condition has reached a predetermined time period. Thus, the sensor element is heated by the heater for the predetermined time period with the temperature of the sensor element and the atmosphere in which the sensor element is operating being suitable for burning impurities attached to the sensor element. Therefore, it is possible to fully burn and positively eliminate the impurities. Further, since the operation of the heater is stopped when the predetermined time period is reached, it is possible to operate the heater in a just appropriate fashion without excessiveness, and thereby minimize power consumption of the heater.
Preferably, the heater control means causes the heater to operate during stoppage of the engine.
According to this preferred embodiment, the heater is also operated during stoppage of the engine. Therefore, even when the engine has stopped before the impurities have been eliminated by operation of the heater during operation of the engine, it is possible to positively eliminate impurities attached to the sensor element. It should be noted that the stoppage of the engine includes stoppage by an idle stop, i.e. a function of a vehicle for automatically stopping operation of the engine e.g. during waiting at a stoplight. Vehicles having an idle stop feature tend to repeatedly carry out operation and stoppage of their engines at short time intervals, and hence there is a high possibility that the idle stop is executed before completion of elimination of impurities. Therefore, this preferred embodiment makes it possible to efficiently obtain the above-described advantageous effects of the invention.
Preferably, an adsorbent for adsorbing hydrocarbons in exhaust gases is arranged within the intake pipe, and the humidity sensor is arranged in the vicinity of the adsorbent, the humidity sensor being used for determination of degradation of the adsorbent, which is executed based on a result of detection by the humidity sensor after the engine has stopped.
According to this preferred embodiment, the determination of degradation of the adsorbent is carried out after the engine has stopped, i.e. based on the result of detection of the upstream humidity sensor executed in a state in which no exhaust gases are flowing. Thus, the determination of degradation of the adsorbent can be carried out without causing the humidity sensor to be exposed to exhaust gases flowing during operation of the engine. This enables the heater control to be carried out during operation of the engine, so that the temperature of the sensor element can be maintained at a predetermined temperature. This makes it possible to suppress attachment of impurities to the sensor element, whereby an excellent detection accuracy of the humidity sensor can be maintained. Further, since the determination of degradation of the adsorbent can be carried out based on the result of detection by the humidity sensor executed in a condition suppressing attachment of impurities to the sensor element, the determination can be carried out with accuracy.
More preferably, a changeover valve is arranged in the exhaust pipe for switching the exhaust pipe between a main passage and a bypass passage bypassing the main passage, and the humidity sensor is arranged in the bypass passage, the changeover valve being configured to switch the exhaust pipe to the main passage during operation of the engine, except when the hydrocarbons are adsorbed by the adsorbent.
According to this preferred embodiment, in the exhaust pipe, there are provided a main passage and a bypass passage bypassing the main passage, and the exhaust pipe is switched to the main passage during operation of the engine, except when the hydrocarbons are adsorbed by the adsorbent. Thus, during operation of the engine, exhaust gases are caused to flow through the main passage except when the hydrocarbons are absorbed by the adsorbent. This makes it possible to prevent the humidity sensor from being exposed to exhaust gases flowing during operation of the engine. Therefore, it is possible to suppress attachment of impurities to the sensor element.
More preferably, the determination of degradation of the adsorbent is carried out on condition that the engine was operating in a predetermined operating condition before stoppage of the engine.
According to this preferred embodiment, the determination of degradation of the adsorbent is carried out after the engine operating in a predetermined operating condition stopped. In general, the adsorbent desorbs hydrocarbons adsorbed thereby when the temperature thereof becomes equal to or higher than a predetermined temperature. Therefore, by setting the predetermined condition such that the temperature of the adsorbent is high enough to desorb the hydrocarbons, it is possible to carry out the determination of degradation of the adsorbent without being affected by hydrocarbons remaining in the adsorbent. This can increase the accuracy of the determination of degradation of the adsorbent.
Further preferably, the predetermined operating condition of the engine is a condition in which the engine operates with supply of a mixture at or in the vicinity of a stoichiometric air-fuel ratio.
According to this preferred embodiment, the determination of degradation of the adsorbent is carried out after the engine operating with supply of a mixture at or in the vicinity of the stoichiometric air-fuel ratio has stopped. When the air-fuel ratio of the mixture is at or in the vicinity of the stoichiometric air-fuel ratio, the amount of unburned components in exhaust gases is more steady or substantially fixed than when the air-fuel ratio of the mixture is on a richer or leaner side, and hence the determination of degradation of the adsorbent is accurate when it is carried out after the engine has stopped after operating with supply of a mixture at or in the vicinity of the stoichiometric air-fuel ratio. Moreover, when the air-fuel ratio of the mixture is at or in the vicinity of the stoichiometric air-fuel ratio, the amount of moisture in exhaust gases is relatively large with little variation, and hence the atmosphere in which the humidity sensor is operating after the engine operated in such a condition and then stopped is also relatively high in humidity with little variation, which is suitable for the determination of degradation of the adsorbent. Therefore, the degradation determination can be accurately carried out in this state.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.